1. Field of the Invention
The present invention generally relates to physical processes such as manufacturing processes for which tools are used and, more particularly, to operator-supervised automated processes for which a plurality of suitable tools may be concurrently available such as lithography or other semiconductor manufacturing processes.
2. Description of the Prior Art
Many physical processes such as those used in manufacturing require the use of tools in order to accomplish the desired result. Often, the tools are specially designed and constructed or adapted for particular processes. Lithographic exposure tools and plasma reactor vessels (either of which may be of any one of many different designs, capacities, efficiencies and the like) are examples of expensive, special-purpose tools used for semiconductor manufacturing processes. However, it is to be understood that many other examples of physical processes and tools for performing them exist and may be of substantially greater or lesser cost and complexity. Box-end and open-end wrenches (which could be used interchangeably for most purposes of assembly of machinery) would be an extremely simple example of tools of different design which might be used for a given process.
Whether the process or the tool is simple or highly complex, there is a tendency for the same tool to be repeatedly used to carry out a given process. In a simple case, an artisan may be likely to choose a tool which most recently provided satisfactory results or a tool with which the artisan is most experienced and comfortable. Likewise, an artisan may choose a tool which is, correctly or incorrectly, thought to be in the best condition even though the condition of the tool may tend to deteriorate during use, such as a tool having a blade for woodworking.
The same is true for complex, automated processes which are supervised by a human operator. For example, if tools having different throughputs are available, an operator is likely to choose the one having the greater throughput and to continue use of the same tool thereafter. Even among a plurality of tools of identical design and specification, the tool of choice is likely to be that which most recently was satisfactorily used.
In critical manufacturing processes, such as those involved in semiconductor integrated circuit manufacture, however, such a tendency to repeatedly choose the same tool for repeated use may have undesired consequences. For example, a need to adjust, calibrate, repair or rework a tool may only become evident after numerous other processes have been completed and/or the process carried out for a relatively large number of wafers in one or more production runs. Further, an unsuitable condition of a tool may not be evident or even easily determinable except through use and, while it may appear that several interchangeable tools are available, a given tool may not, in fact; be in satisfactory condition or a substituted tool may not provide satisfactory results or results that are complementary to or compatible with other processes within a manufacturing line.
At the same time, however, it is desirable in critical manufacturing processes such as semiconductor manufacturing, to maintain as high a degree of consistency as possible to minimize variation in the final product. It is not possible, even on identical tools, to exactly maintain all processing parameters unchanged. (Even though process variation can be quite small on a given tool, process variation can occur for different locations within the tool in a single process such as between wafers concurrently processed using the tool or even across a single wafer. For example, astigmatism across the field of a lithographic exposure tool can compromise otherwise identical exposures made in a step-and-repeat fashion. Similarly, wafer chucks must be carefully designed to limit temperature variation across a wafer to minimize variation in growth, deposition or etch rates.)
Such variation in process parameters, even on a single tool, may use a significant portion of the so-called "process window" for a given process and closely matching performance on a different tool is often obtained only with difficulty. Therefore, while there are advantages in manufacturing yield and product consistency through consistent use of a particular tool, a change of condition of the tool may severely compromise a large segment of a production run if unsuitable operating conditions are not detected in a timely fashion and if one specific tool is used for all devices in a production run, a set of jobs or lots, or an entire manufacturing line since a replacement tool may not, as a practical matter, be available to substitute for another tool taken out of service.
Additionally, the substitution of one tool for another in a production line may carry economic costs such as the time necessary to implement the substitution, change of throughput and the like while the condition of any tool may only be determinable through at least periodic use. Therefore, significant practical trade-offs exist between consistent use of a particular tool and use of a plurality of tools in sequence or alternation to carry out a process.
Another consideration alluded to above is the service requirements or usable lifetime of any particular tool or part thereof. Usually, the number of hours of use or the number of times a process can be performed prior to a need arising for maintenance or service of the tool can be projected with adequate accuracy to maintain overall efficiency and avoid down-time of the production line. However, the number of processes which can be carried out between maintenance operations can be strongly affected by both the nature of the process performed and the process parameter tolerances within the product. That is, how much process parameter variation attributable to use of the tool can be tolerated before manufacturing yield is compromised.
Tool performance can also be degraded by either frequency of use or period of non-use and any change in performance with use must be considered against the cost of establishing necessary conditions of the tool, such as bringing a reactor vessel to the proper temperature. The impact of a need for tool maintenance will also vary with the circumstances of the production line (e.g. the economic cost of stopping a production line for the period of maintenance or to substitute another tool).
Finally, when a tool or tool set is placed back into service, the only way to verify the success or failure of any changes made to the tool is to utilize the tool in production. Although resulting variations in product are thus unavoidable, every effort must be made to keep the entire product set from flowing through the tool in question until a final product certification can be completed.
Currently, there is no systematic methodology for relating tool use, process condition change with tool usage pattern and tool maintenance with product requirements. Moreover, merely alternating or sequencing use of tools in a tool set (e.g. a group of tools suitable to a particular process) is often inconsistent with efficiency and high levels of productivity. Further, for complex processes such as in semiconductor manufacturing where the design of the product is strongly dependent on tool capability, new designs of tools are being consistently produced to improve process accuracy, throughput and the like. However, such tools must be qualified as suitable for a given process before they are placed in a manufacturing line. At the same time, such tools are often installed proximate to a manufacturing line to be used experimentally and for developing calibration and other aspects of use before being used to produce product which may be sold but could be placed on-line at the will of an operator. Currently, there is no procedure for preventing an operator from placing an unqualified tool in a manufacturing line.
However, an operator is responsible for the productivity of a manufacturing line which includes one or more tools, such as those described above. Therefore, productivity should be of paramount importance and consideration of tool condition or balancing of tool use may be a significant distraction. The operator must have the flexibility to choose among functional and qualified tools to maintain production volume of high-quality product. At the same time, tool condition and balancing of tool use must be considered to maintain manufacturing yield and to insure the integrity of the tools in the manufacturing line, including the availability of replacement tools, when needed, to avoid lengthy down-time of the manufacturing line.